The present invention pertains to an organic electroluminescent devices and more specifically to a structure for improving the efficiency and lifetime of an organic electroluminesocnt device.
In recent progress of the organic electroluminescent device, the organic EL devices are attractive owing to the merits of high brightness, wide viewing angle, low driving voltage and capability for full color flat portable emissive displays. The normal organic electroluminescent device composes multi-layers of thin film sandwiched between two electrodes. The organic layer composes hole transporting layer, light emission layer and electron transporting layer. Either the electron transporting layer or the hole transporting layer can be designed as the emissive layer and the light can be transmitted either way but generally exits through one of the conductive layers. There arm many methods to modify one of the conductive layers for the emission of light there through but it has been observed that the most efficient organic electroluminescent device includes one conductive layer, which is transparent to the light being emitted. The widely used material for this conductive and transparent layer is indium-tin-oxide (ITO), which is usually deposited on a transparent substrate, for example, the glass or plastic plate.
Although the multi-layer structure is normally used in constructing the organic electroluminescent devices, the interface between the electron transporting layer and the hole transporting layer is not compatible and results in a bad junction in the interface of the different layers and eventually causes the stripped off phenomenon in the device under high temperature condition. Also the lifetime of multi-layered organic electroluminescent devices is influenced by the abrupt change of the interface between the hole and electron transporting layers when the organic EL device is under voltage bias.
Significant efficiency change of the electroluminescent has been achieved in the prior art (See U.S. Pat. No. 5,925,980). In this the patent, a structure of electroluminescent is provided which comprises a hole transporting region, an electron transporting region and a graduated region disseminated between the hole transporting region and the electron transporting region is provided. The graduated region changes, either in steps or continuously, from the hole transporting organic material adjacent to the hole transporting region to the electron transporting organic material adjacent to the electron transporting region. Further improvement is still needed for applications where lifetime is a primary concern.
The lifetime of an organic electroluminescent is affected by the stability of both the bulk morphology of the hole transporting materials and the interface between the hole and electron transporting layers when the organic electroluminescent is applied bias.
Several schemes have been proposed to address the problem of bulk morphology stability of the hole transporting material in an organic electroluminescent. Among other things, the lifetime of the electroluminescent device is improved by the elimination of heterojunction in a continuous organic medium (See U. S. Pat No. 6,130,001). In the patent, it provides an organic electroluminescent layer which comprises a continuous organic medium AxBy, where A and B an components capable of transporting holes and electrons, respectively and x represents the content of A component with a value ranging form 0 adjacent to the anode to 100% adjacent to the cathode, and y represents the content of B component with a value ranging from 0 adjacent to the cathode to 100% adjacent to the anode. The lifetime of the device is thus improved by the elimination of heterojunctions in the continuous organic medium.
Generally, there is always a need to provide a smooth reliable region so that the interface effect can be reduced to a minimum. However, the smooth reliable region should not come at the expense of the lifetime and efficiency of the organic electroluminescent device. Also, the elimination of the barrier in the interface between different layers will give the electroluminescent device with great improvement in reliability and life time.
It is a purpose of the present invention to provide a new and simplified organic electroluminescent device with an improved lifetime.
It is another purpose of the present invention to provide a new and simplified organic electroluminescent device with high efficiency.
It is still another purpose of the present invention to provide a new and simplified organic electroluminescent device with improved stability.
The above problems and others are at least partially solved and the above purposes and others are realized in an organic electroluminescent device including an emitting layer comprising AxByCz inserted between the anode and the cathode, and a transporting auxiliary layer is inserted between the emitting layer and the cathode. The emitting layer with different materials are mixed in a single layer which can solve the normally observed interface problem. Moreover, the transporting auxiliary layer comprises LiF (electron injecting material) alone or a mixture of the hole transporting material and the electron transporting material AxBy and an a % wt of LiF. The purpose of the transporting auxiliary layer is used mainly to enhance the transporting performance efficiency of both electron and hole in the electroluminescent device.
The emitting layer AxByCz comprises a conventional hole transporting material Ax, a new electron transporting material By, and at least a new non-dopant emitting material or pigment Cz incorporated in the organic emitting layer and x, y, z denotes the content of the medium A, B, C respectively. While the transporting auxiliary layer comprises LiF alone or AxBy and an a % wt of LiF mixture. The three materials used for forming the electroluminescent device (A, B, and LiF) are mixed evaporated according to a specific combination ratio in the chamber to form the new structure of the emitting device.
Furthermore, the content x, y, and z in medium A, B, and C in both of the emitting layer and the transporting auxiliary layer have the following characteristics: the content x of medium A is gradually reduced from 100% adjacent to the anode to 0% adjacent to the cathode, the content y of medium B is gradually increased from 0% adjacent to the anode to 100% adjacent to the cathode, while the content z of the non-dopant emission material C is gradually increased from 0% adjacent to the anode to 100% adjacent to the transporting auxiliary layer.